This application relates to a method of desizing or removing an organic coating from ceramic or glass fibers and using remnants of the removal process as a reactant to provide an interface coating.
Ceramic and glass fibers are utilized in the formation of ceramic matrix composite (“CMC”) materials. CMC materials are often utilized in high temperature applications. As an example, gas turbine engines may incorporate a number of components formed of CMC materials. CMC materials are typically formed from a process that includes the steps of: lay-up and fixation of ceramic fibers to form a preform; infiltration of a matrix material into the preform to form a CMC structure; final machining of the CMC structure; and any other further treatments, such as providing additional coatings for example.
The CMC materials are formed from high temperature ceramic or glass fibers, such as silicon carbide (SiC) fibers, siliconborocarbonitride (SiBCN) fibers, silicon nitride (Si3N4) fibers, boron carbide (B4C) fibers, etc. The diameter of such fibers may range between 5 and 150 microns. These fibers are typically provided with a polymeric fiber coating, also called a sizing, which prevents fiber breakage and damage during handling after production.
Additionally, interface coatings (IFC) are needed on the ceramic fibers to provide a weak interface for debonding from the matrix, as well as offering environmental protection of the fiber. As an example, an interface coating for silicon based CMCs would be a boron nitride or pyrolytic carbon inner layer with an additional outer layer such as SiC or Si3N4, for example.
During the fabrication of CMCs, the sizing must be removed from the fibers, or desized, prior to application of the fiber interface coating(s). Typically, the fibers are heated to burn off the sizing, which can form residual carbon on the surface of the fibers. Traditionally, the desizing of fibers, such as SiC for example, has been provided by using a high temperature ammonia heat treatment. In some instances, other methods such as passing the fibers through a flame or other oxidizing atmosphere have also been used to burn off the polymer coating and desize the fiber. For textile sizes and applications, hot water treatments have also been utilized. It is important that the sizing be completely removed prior to application of the interface coating to ensure that the final CMC structure has the desired properties.